The present invention relates generally to integrated circuit devices and, more particularly, to a system and method for implementing a micro-stepping delay chain for a digital delay locked loop (DLL).
In synchronous electronic systems, the integrated circuits in the system are synchronized to a common reference clock. This synchronization often cannot be achieved simply by distributing a single reference clock to each of the integrated circuits for the following reason, among others. When an integrated circuit receives a reference clock, the circuit often must condition the reference clock before the circuit can use the clock. For example, the circuit may buffer the incoming reference clock or may convert the incoming clock from one voltage level to another. This processing introduces its own delay, with the result that the processed reference clock, which will be referred to as a local clock, often will no longer be adequately synchronized with the incoming reference clock. The trend towards faster system clock speeds further aggravates this problem since faster clock speeds reduce the amount of delay, or clock skew, which can be tolerated.
To remedy this problem, an additional circuit is typically used to synchronize the local clock to the reference clock. Two common circuits that are used for this purpose are the phase-locked loop (PLL) and the delay-locked loop (DLL). In the phase-locked loop, a voltage-controlled oscillator produces the local clock. The phases of the local clock and the reference clock are compared by a phase-frequency detector, with the resulting error signal used to drive the voltage-controlled oscillator via a loop filter. The feedback via the loop filter phase locks the local clock to the reference clock. Stability of the feedback loop, however, depends in part on the loop filter. The electronic characteristics of the loop filter, in turn, often depend significantly on manufacturing parameters. As a result, the same loop filter design may result in a stable feedback loop when manufactured with one process but an unstable loop when manufactured by another. It is difficult to produce a single loop filter design for use with all manufacturing processes, and the design of the loop filter typically must be optimized on a process-by-process basis.
In contrast, the delay-locked loop generates a synchronized local clock by delaying the incoming reference clock by an integer number of clock periods. More specifically, the buffers, voltage level converters, etc., of the integrated circuit introduce a certain amount of delay. The DLL introduces an additional amount of delay such that the resulting local clock is synchronous with the incoming reference clock. This approach avoids the stability problem inherent in the phase-locked loop approach. Delay-locked loops, however, have a disadvantage of narrow dynamic range relative to their precision. That is, a highly accurate DLL requires that the two clock signals being synchronized have a phase difference that is relatively small. On the other hand, conventional DLLs can be made to synchronize clocks with a larger phase difference, but the resulting accuracy decreases and may be less than desirable.
Accordingly, there is a need for an improved DLL device which synchronizes local clocks to reference clocks, and which provides a wider dynamic range of operation with acceptable precision.